Design, simulation, fabrication, and characterization of a digital variable optical attenuator

In this paper, we present the design, simulation, fabrication, and some measurement and characterization of a novel 16-bit digital variable optical attenuator (VOA) that attenuates by switching individual mirror of an array as an attempt to achieve input voltage variation independence and output linearization. The design was aided by a simulation package that features coupled electrostatic and mechanical solver. The mirror array spans an area of 1500 × 1500 μ m² and contains 16 equal-length rectangular micromirrors. Each mirror is suspended by two torsion beams. Experiments on beam design and width variations are conducted. Assuming Gaussian distribution, the mirror widths computed by an iteration algorithm vary from about 40 to 250 μm. Based on silicon-on-insulator (SOI) technology, two fabrication schemes to open the backside optical entrance were investigated. A hydrofluoric (HF) acid vapor-phase-etching (VPE) setup built to release the microstructure anhydrously is the key to achieve high yield especially for fragile components. Surface flatness, resonance frequencies, and tilt angles of selected mirrors were characterized. Quartz chips patterned with aluminum electrodes and 10 μm-high SU8 spacer columns were fabricated and assembled to corresponding device chips. Optical performance adversely affected by mirror bending is believed to originate from the intrinsic stress of the SOI wafer.